Radio communication systems rely on modulating carrier frequencies in a finite portion of the electromagnetic spectrum to wirelessly transmit and receive signals. Modulation can be performed on the amplitude, frequency, and/or phase of the carrier frequency to separate the signal from unwanted noise. The signals typically convey various types of information such as audio, video, and data to and from transceiving devices such as cellular base stations, cellular subscriber units, and personal computers.
In code division multiple access communication systems, a plurality of transmissions, each with its own code, can share the same band of frequencies. Each transmission is spread over the available bandwidth by mixing the signal to be transmitted with that signal's unique code. For a CDMA system, spreading gain is defined as the ratio of chip rate to information data rate. This ratio also quantifies the redundancy for any particular user. Typically, information data stream is first encoded by a forward error correction (FEC) code such as a convolutional code. The FEC encoded data stream then is further spreaded by a code from an orthogonal codeword set.
In the forward link (the link from base station to the mobile station), prior art systems assign each user a single codeword from the orthogonal codeword set. By alternating transmission spreading between the assigned orthogonal code and its compliment, information is conveyed from a transmitter to a receiver. Since different transmitters use distinct codewords, the receiver can correlate the designated transmitter's orthogonal code to retrieve its information and minimize interference. Therefore, in the forward link, the orthogonal code has two functions, spreading and channelization. In spite of the large redundancy used by the spreading, this conventional orthogonal spreading/channelization does not provide any coding gains.